1) Preheat the oven to 350°F.
2) Line a 9" x 12" pan with waxed paper and grease it.
3) In a saucepan, combine cocoa, white sugar and hot water.
4) Place the saucepan, over low heat to cook.
5) Beat in egg yolk and vanilla into the sauce. Place the saucepan aside to cool.
6) In a bowl, cream the butter and sugar.
7) Add eggs and egg white to beat in.
8) Fold in cooled chocolate sauce into the egg mixture.
9) Mix buttermilk by adding the baking soda.
10) In another bowl, sift together cake flour and baking powder.
11) Alternately, add buttermilk and flour mixture to the egg mixture.
12) Use a 9" x 12" pan lined with waxed paper, to spread the batter.
13) Place inside oven to bake for 45 minutes.
14) Cool the cake over rack.
15) Ice the cake with your choice of icing and serve.